Disc brake pads are an example of a laminate where a friable pad of friction material is bonded to a backing plate of ductile steel. The plate lamina backs the friable friction lamina, preventing it from cracking or fracturing. Concrete, cement, gypsum board and the like are examples of friable materials that may likewise be laminated to more ductile materials. In this disclosure, the brake pad will be used as the exemplary laminate.
Brake pads are subject to enormous stress. High shear forces combine with extreme range of temperature, and, severe environmental exposure. Yet they are fundamental to safe vehicle operation.
By way of background, today's brake pads are mould-bonded where a mixture of assorted powdered minerals and resins along with organic and metallic fibres is compressed in a mould onto a bare, steel backing plate. With heat, the resin first melts to wet the plate and permeate the mixture and then cures, resulting in a rock-hard friction pad bonded to the plate's surface with a thin layer of adhesive, which is referred to as the bond line.
Mould-bonding is a slow, inefficient batch process where friction powder is compressed onto bare steel backing plates set in moulds. Batches of filled moulds are then pressure-heated, cooled, and unloaded, and then cleaned for reuse.
During braking, especially with a heavy and/or fast moving vehicle, the hydraulic caliper squeezes the brake disc with the brake pads resulting in frictional drag that generates high temperatures and high shear forces at the bond line.
Over time, moisture reaches the bare steel which begins rusting. The rust, in turn, progressively weakens the bond line which can lead to partial or full delamination, a highly dangerous condition.
An electro-plated or otherwise protectively coated steel backing plate is not suitable for the mould-bonding process because the process requires a clean, bare steel surface.
Friction pads used to be riveted to plated steel. However, the holes led to fracturing-crumbling-spalling of the pad, escalating with wear. If rivet heads wore off, the unsecured friction made for dangerous conditions. As a result rivets have largely been replaced by the above described mould-bonded brake pad and its aforementioned rust problem.